1. Field of the Invention
The present invention relates generally to an optical amplifier for wavelength-division multiplexing, and more particularly to an optical amplifier suitable for a terminal for transmission.
To in crease the capacity of an optical communication system, research into wavelength-division multiplexing (WDM) has widely been pursued. For example, by combining WDM and an erbium doped fiber amplifier (EDFA), a large-capacity and long-haul optical communication system can be constructed.
2. Description of the Related Art
Conventionally known is an optical communication system including a first terminal for transmission, a second terminal for reception, and an optical fiber transmission line laid between the first and second terminals. In the case that WDM is applied to this system, the first terminal includes a plurality of optical transmitters for respectively outputting optical signals having different wavelengths and an optical multiplexer for wavelength-division multiplexing the optical signals to generate WDM signal light. The optical multiplexer has at least one output port for outputting the WDM signal light. Accordingly, the WDM signal light can be output to at least one optical fiber transmission line. In the system adopting WDM in this manner, a transmission capacity per line can be increased, thereby allowing a large capacity of the optical communication system.
In recent years, research and development on application of an optical amplifier to an optical communication system have intensively been pursued. For example, the importance of a postamplifier, optical repeater, and preamplifier each having an EDFA has become apparent.
Conventionally known is an optical amplifier including an optical amplifying medium and means for pumping the optical amplifying medium so that the optical amplifying medium has a gain band including the wavelengths of optical signals. In the case that the optical amplifying medium is an EDF (erbium doped fiber) having a first end and a second end, the pumping means includes a pump light source for outputting pump light having a proper wavelength, and means for supplying the pump light into the EDF from at least one of the first end and the second end. In the case that the optical amplifying medium is provided by a semiconductor chip, the pumping means includes means for injecting a current into the chip.
In some case, a dispersion compensator is provided in the first or second terminal or inserted in the optical fiber transmission line, so as to compensate for dispersion occurring in the optical fiber transmission line. Known examples of the dispersion compensator include a dispersion compensating fiber (DCF) having dispersion opposite to the dispersion in the optical fiber transmission line, and the combination of a fiber grating having a proper distribution of grating pitches and an optical circulator.
For example, in the case that the dispersion compensator employing the DCF is provided in the first terminal for transmission, a plurality of DCFs are respectively used for a plurality of channels of optical signals, because there is a difference in amount of dispersion compensation, that is, length of the DCF to be required between the channels of the optical signals. In this case, in configurating a postamplifier by inserting an optical amplifier between the optical multiplexer and the optical fiber transmission line, input powers to the postamplifier vary between the channels. Therefore, a wide input dynamic range is required for the optical amplifier. The variations in input powers are due to the fact that a loss per unit length of each DCF is large and the difference in length between the DCFs directly causes a difference in loss.
Further, an optical amplifier has a gain tilt in general. Therefore, in the case that there is a deviation in wavelength of each optical signal, the output power from the optical amplifier is deviated according to this deviation. For example, in an EDFA, it is known that the gain tilt occurs in a saturated region being driven by pump light having a relatively high power.